This invention relates to an apparatus for controlling an adjustable-speed alternating current (AC) motor coupled to drive an elevator cage.
Electric power converters or inverters have been employed for the application of adjustable-speed drives using alternating current motors. A typical converter includes a direct current (DC) rectifier for rectifying three-phase AC input voltage and for supplying the resulting DC bus potential to an inverter. The inverter comprises a plurality of pairs of series-connected switching elements to generate an adjustable frequency output. In many applications, such a frequency adjustment is effected through a control circuit which employs a pulse width modulation (PWM) control technique in producing variable frequency gating pulses to periodically switch the respective switching elements so as to operate the AC motor at a variable speed. The AC motor can be propelled (motoring mode) or retarded (braking mode) as desired by appropriately varying the frequency and the amplitude of the excitation that the inverter applies to the AC motor. The AC motor operation is changed frequency between the motoring and braking modes particularly when the AC motor is used to drive an elevator. It is necessary to flow power from the AC motor back through the inverter to the converter during the braking mode or other regenerative conditions in order to operate the AC motor with high efficiency.
It is the conventional practice to bring the elevator to a stop by applying a mechanical braking while stopping the inverter operation when a power failure occurs. However, this requires a large-sized mechanical braking system which can absorb the whole inertia energy of the elevator. This is true particularly for large-sized and high-speed elevators. In addition, there is no way to handle the regenerated power when the regenerative function is inhibited in response to the power failure.